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BIOLOGICAL SCIENCES / PSYCHOLOGY
Inequity responses of monkeys modified by effort
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*Living Links, Yerkes National Primate Research Center, Emory University, Atlanta, GA 30329; and Department of Anthropology, Emory University, Atlanta, GA 30322
Contributed by Frans B. M. de Waal, August 17, 2007 (received for review May 14, 2007)
Without joint benefits, joint actions could never have evolved. Cooperative animals need to monitor closely how large a share they receive relative to their investment toward collective goals. This work documents the sensitivity to reward division in brown, or tufted, capuchin monkeys (Cebus apella). In addition to confirming previous results with a larger subject pool, this work rules out several alternative explanations and adds data on effort sensitivity. Thirteen adult monkeys exchanged tokens for rewards, showing negative reactions to receiving a less-favored reward than their partner. Because their negative reaction could not be attributed to the mere visibility of better rewards (greed hypothesis) nor to having received such rewards in the immediate past (frustration hypothesis), it must have been caused by seeing their partner obtain the better reward. Effort had a major effect in that by far the lowest level of performance in the entire study occurred in subjects required to expend a large effort while at the same time seeing their partner receive a better reward. It is unclear whether this effort–effect was based on comparisons with the partner, but it added significantly to the intensity of the inequity response. These effects are as expected if the inequity response evolved in the context of cooperative survival strategies.
capuchin monkey | Cebus apella | cooperation | frustration | fairness
The authors declare no conflict of interest.
¶ N varies because ties are excluded from Wilcoxon tests.
|| The typical exchange measure employed in the present work differs from the measure used in ref. 1, which concerned refusals to exchange or to consume rewards. Using the latter measure, subjects showed fewer refusals in the Eq than the Ineq test (Wilcoxon Eq vs Ineq: n = 9, T = 6, P < 0.05, one-tailed). The Eq-G test showed an intermediate outcome, not significantly different from either the Eq or Ineq test (Wilcoxon Eq vs. Eq-G: n = 9, T = 11, NS; Ineq vs. Eq-G: n = 11, T = 26, NS, both one-tailed). This result may mean that greed, or the desire for a better reward, plays a somewhat stronger role in whether subjects complete the interaction at all rather than in a typical fashion, although these results also suggest that inequity is a stronger factor than greed.
To whom correspondence may be sent at the present address: Department of Psychology, Georgia State University, P.O. Box 5010, Atlanta, GA 30302-5010. E-mail: sbrosnan{at}gsu.edu.
To whom correspondence may be addressed. E-mail: dewaal{at}emory.edu
© 2007 by The National Academy of Sciences of the USA
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